This patch adds support for JPEG decoding. The JPEG decoder is capable
of handling standard 2x1 horizontal, 2x1 vertical and quartered chroma
subsampling. The implemented Inverse DCT performs with a decent speed.
As of interchange formats, since we tend to ignore the metadata in APPn
markers, the decoder can handle any format compatible with JFIF, which
includes EXIFs and sometimes WebMs too. The decoder does not support
progressive JPEGs yet.